前言
❝
图层加载具有生命周期，什么时候开始加载？什么时候正在加载？什么时候加载完成？在图层加载过程中，经常需要判别图层是否加载完成。在
OpenLayers如何查看图层加载进度呢？
1. 基本原理
要实现显示图层加载进度可以通过监听图层数据源事件来完成，像矢量数据源具有[featuresloadstart](https://openlayers.org/en/latest/apidoc/module-ol_source_Vector.VectorSourceEvent.html#event:featuresloadstart)、[featuresload](https://openlayers.org/en/latest/apidoc/module-ol_source_Vector.VectorSourceEvent.html#event:featuresloadstart)end两个事件进行处理。[featuresloadstart](https://openlayers.org/en/latest/apidoc/module-ol_source_Vector.VectorSourceEvent.html#event:featuresloadstart)事件对应要素开始加载阶段，[featuresload](https://openlayers.org/en/latest/apidoc/module-ol_source_Vector.VectorSourceEvent.html#event:featuresloadstart)end事件对应要素加载结束事件。并在事件监听过程中修改计数器的值。
2. 创建进度条构造函数
进度条构造函数初始参数为targetEle, countEle，分别是进度条目标要素与计数器要素。在进度条原型上添加更新进度条和计数器方法和显示、隐藏进度条的方法。
/**
* 控制进度条构造函数
* @targetEle：进度条要素
* @countEle：计数器要素
*/
function Progress(targetEle, countEle) {
this.el = targetEle
// 加载总数
this.loading = 0
// 已加载数
this.loaded = 0
this.countEle = countEle
}
// 更新进度条
Progress.prototype.addLoading = function () {
++this.loading
this.update()
}
// 更新加载数量和计数器
Progress.prototype.addLoaded = function () {
++this.loaded
++this.countEle.textContent
this.update()
}
// 更新进度条
Progress.prototype.update = function () {
const width = ((this.loaded / this.loading) * 100).toFixed(1) + "%"
this.countEle.textContent = width
this.el.style.width = width
}
// 显示进度条
Progress.prototype.show = function () {
this.el.style.visibility = 'visible'
}
// 隐藏进度条
Progress.prototype.hide = function () {
const style = this.el.style
// 延迟隐藏
setTimeout(function () {
style.visibility = 'hidden'
style.width = 0
},250)
}
3. 监听数据源加载
在featuresloadstart要素开始加载事件中更新进度条，在featuresloadend要素结束加载事件中更新进度条和计数器。
vectorSource.on("featuresloadstart", function () {
progress.addLoading()
})
vectorSource.on("featuresloadend", function () {
progress.addLoaded()
})
4. 监听地图事件
在地图开始加载loadstart事件中显示进度条，在地图加载结束loadend事件中隐藏进度条。
map.on('loadstart', function () {
progress.show()
})
map.on('loadend', function () {
progress.hide()
})
5. 完整代码
其中libs文件夹下的包需要更换为自己下载的本地包或者引用在线资源。
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>OpenLayers 图层加载进度条</title>
<meta charset="utf-8" />
<link rel="stylesheet" href="../../libs/css/ol9.2.4.css">
<link rel="stylesheet" href="../../libs/layui/css/layui.css">
<script src="../../js/config.js"></script>
<script src="../../libs/js/ol9.2.4.js"></script>
<script src="../../libs/layui/layui.js"></script>
<style>
* {
padding: 0;
margin: 0;
font-size: 14px;
font-family: '微软雅黑';
}
html,
body {
width: 100%;
height: 100%;
}
#map {
position: absolute;
top: 50px;
bottom: 0;
width: 100%;
background: #E0ECED;
}
#top-content {
position: absolute;
width: 100%;
height: 50px;
line-height: 50px;
background: linear-gradient(135deg, #ff00cc, #ffcc00, #00ffcc, #ff0066);
color: #fff;
text-align: center;
font-size: 32px;
}
#top-content span {
font-size: 32px;
}
.load-count {
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
width: 100px;
height: 100px;
border-radius: 50%;
background-color: #00ffcc;
color: #fff;
text-align: center;
line-height: 100px;
}
.progress-bar {
position: absolute;
bottom: 0;
left: 0;
height: 2px;
background: rgba(3, 3, 150);
width: 0;
transition: width 250ms;
}
</style>
</head>
<body>
<div id="top-content">
<span>OpenLayers 图层加载进度条</span>
</div>
<div id="map"></div>
<div class="load-count">0</div>
<div class="progress-bar"></div>
</body>
</html>
<script>
/**
* 控制进度条构造函数
* @targetEle：进度条要素
* @countEle：计数器要素
*/
function Progress(targetEle, countEle) {
this.el = targetEle
// 加载总数
this.loading = 0
// 已加载数
this.loaded = 0
this.countEle = countEle
}
// 更新进度条
Progress.prototype.addLoading = function () {
++this.loading
this.update()
}
// 更新加载数量和计数器
Progress.prototype.addLoaded = function () {
++this.loaded
++this.countEle.textContent
this.update()
}
// 更新进度条
Progress.prototype.update = function () {
const width = ((this.loaded / this.loading) * 100).toFixed(1) + "%"
this.countEle.textContent = width
this.el.style.width = width
}
// 显示进度条
Progress.prototype.show = function () {
this.el.style.visibility = 'visible'
}
// 隐藏进度条
Progress.prototype.hide = function () {
const style = this.el.style
// 延迟隐藏
setTimeout(function () {
style.visibility = 'hidden'
style.width = 0
}, 250)
}
//地图投影坐标系
const projection = ol.proj.get('EPSG:3857');
//==============================================================================//
//============================天地图服务参数简单介绍==============================//
//================================vec：矢量图层==================================//
//================================img：影像图层==================================//
//================================cva：注记图层==================================//
//======================其中：_c表示经纬度投影，_w表示球面墨卡托投影================//
//==============================================================================//
const TDTImgLayer = new ol.layer.Tile({
title: "天地图影像图层",
source: new ol.source.XYZ({
url: "http://t0.tianditu.com/DataServer?T=img_w&x={x}&y={y}&l={z}&tk=" + TDTTOKEN,
attibutions: "天地图影像描述",
crossOrigin: "anoymous",
wrapX: false
})
})
const TDTImgCvaLayer = new ol.layer.Tile({
title: "天地图影像注记图层",
source: new ol.source.XYZ({
url: "http://t0.tianditu.com/DataServer?T=cia_w&x={x}&y={y}&l={z}&tk=" + TDTTOKEN,
attibutions: "天地图注记描述",
crossOrigin: "anoymous",
wrapX: false
})
})
// 文字样式
const labelStyle = new ol.style.Style({
text: new ol.style.Text({
font: '12px Calibri,sans-serif',
overflow: true,
// 填充色
fill: new ol.style.Fill({
color: "#FAFAD2"
}),
// 描边色
stroke: new ol.style.Stroke({
color: "#2F4F4F",
width: 3,
}),
})
})
// 行政区样式
const regionStyle = new ol.style.Style({
fill: new ol.style.Fill({
color: [230, 230, 250, 0.25],
}),
stroke: new ol.style.Stroke({
color: "#00FFFF",
width: 1.25,
}),
})
const style = [labelStyle, regionStyle]
const loadCount = document.querySelector(".load-count")
const progressBar = document.querySelector(".progress-bar")
const progress = new Progress(progressBar, loadCount)
const JSON_URL = "https://geo.datav.aliyun.com/areas_v3/bound/530000_full.json"
const vectorSource = new ol.source.Vector({
url: JSON_URL,
format: new ol.format.GeoJSON()
})
const layer = new ol.layer.Vector({
source: vectorSource,
style: function (feature) {
const label = feature.get("name").split(" ").join("n")
labelStyle.getText().setText(label)
return style
},
declutter: true
})
vectorSource.on("featuresloadstart", function () {
progress.addLoading()
})
vectorSource.on("featuresloadend", function () {
progress.addLoaded()
})
const map = new ol.Map({
target: "map",
loadTilesWhileInteracting: true,
view: new ol.View({
center: [101.485106, 25.008643],
zoom: 6.5,
worldsWrap: false,
minZoom: 1,
maxZoom: 20,
projection: 'EPSG:4326',
}),
layers: [TDTImgLayer, layer],
// 地图默认控件
controls: ol.control.defaults.defaults({
zoom: false,
attribution: true,
rotate: true
})
})
map.on('loadstart', function () {
progress.show()
})
map.on('loadend', function () {
progress.hide()
})
</script>
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